The goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include any trait deemed beneficial or desirable by a grower and/or consumer, including greater yield, resistance to insects or disease, tolerance to environmental stress, and nutritional value.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all genetic loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different genotypes produces a uniform population of hybrid plants that are heterozygous for many genetic loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One breeding technique of value for use in onions is use of “dihaploid” breeding. This technique involves doubling the chromosome number of a haploid derived from a starting heterozygous plant in order to recover a progeny dihaploid plant that is homozygous, with both sets of chromosomes being recovered from the haploid in the case of a diploid species. Polyploids may also be obtained using this method.
One crop species which has been subject to such breeding programs and is of particular value is onion. Onions belong to the lily family, Amaryllidaceae, and the genus, Allium. This genus comprises a group of perennial herbs having bulbous, onion-scented underground leaves, including such commonly cultivated crops as onion, garlic, chives, shallots and leeks. It also includes ornamental species grown for their flowers.
Onions are an important vegetable world-wide, ranking second among all vegetables in economic importance with an estimated value of $6 billion dollars annually. The onion is also one of the oldest cultivated vegetables. Common bulbing onions are of the species Allium cepa. Onions are classified in numerous ways, by basic use, flavor, color, shape of the bulb, and day length. Onions come in white, yellow, and red colors. The bulb may be rounded, flattened, or torpedo shaped.
Commercial onions include “storage onions”, “fresh onions”, “pearl or mini onions”, and “green bunching onions.” Green bunching onions, also called Welsh onions or Japanese bunching onions, are young onion plants that are harvested with the green tops attached and are used both fresh and as a cooked ingredient. They are mild in flavor and are used frequently in salads. These are typically of the species Allium fistulosum. Green bunching onions are available in the United States during the entire year.
Onions are most frequently grown from seed. A small fraction of the crop is grown from transplants or “sets”. Onion sets are produced by sowing seed very thickly one year, resulting in small bulbs that that are easily set out and grown into mature bulbs the next year, Onion transplants are seedlings that are sown thickly and grown without bulbing for 45-90 days. These plants are then harvested and trimmed and replanted at lower populations in the final fields for bulb production. While breeding efforts to date have provided a number of useful onion lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.